Chapter 2 — The Human Eye and the Colourful World
Your eye is a lens that refocuses itself, white light is a mixture of colours, and the air above you bends and scatters sunlight. This chapter explains how we see — and why the world looks the colours it does.
Every picture you have ever seen was made by a convex lens about the size of a lentil. The cornea and eye lens of each eye gather light from the scene in front of you and form a tiny, upside-down image on the retina at the back — and the eye keeps that image sharp whether you look at the stars or at the print on your phone. When the focusing goes slightly wrong, the world blurs, and a pair of spectacles with exactly the right lens power, worked out with the lens formula from Chapter 1, puts it right again.
The second half of the chapter is about colour. A glass prism on a sunny windowsill throws a band of colours on the wall: white light is really a mixture of colours, and glass bends each one by a slightly different amount. Millions of falling raindrops do the same thing to make a rainbow. The air itself bends starlight, so stars twinkle and the Sun is visible a little before it actually rises; and tiny particles in the air scatter blue light more than red, which is why the sky is blue and danger lamps are red. Every one of these effects is refraction (Section 1.3) at work — or light bouncing off things far smaller than a speck of dust.
| What you notice | What the light does | Section |
|---|---|---|
| You can read a book, then look out of the window | the eye lens changes its focal length | 2.1 |
| Spectacles of "$-1.5$" or "$+2.0$" | a lens adds or removes converging power | 2.1 |
| A band of colours from a prism; a rainbow | each colour refracts by a different amount | 2.2 |
| Twinkling stars; a sunrise two minutes early | air layers of different density bend light | 2.3 |
| Blue sky, white clouds, red danger lamps | small particles scatter blue light most | 2.3 |
- name the parts of the human eye and explain how it forms an image on the retina;
- explain accommodation, and state the near point ($25\ \text{cm}$) and far point (infinity) of a normal eye;
- describe myopia, hypermetropia and presbyopia, and calculate the power of the lens that corrects each one;
- trace a ray through a triangular glass prism and identify the angle of deviation;
- explain dispersion of white light into a spectrum, its recombination, and the formation of a rainbow;
- use atmospheric refraction to explain twinkling stars and the early sunrise and late sunset;
- use scattering of light to explain the Tyndall effect, the blue sky, white clouds and red danger signals.
Sections in this chapter
The Human Eye and Defects of Vision
How the eye forms an image, accommodation, near and far points, and the lenses that correct myopia, hypermetropia and presbyopia.
Refraction through a Prism and Dispersion
The path of light through a glass prism, the angle of deviation, the spectrum of white light, recombination and the rainbow.
Atmospheric Refraction and Scattering of Light
Twinkling stars, advance sunrise and delayed sunset, the Tyndall effect, and why the sky is blue and the sunset red.